1. Field of the Invention
The present invention relates to an amplifier circuit used as a balanced modulating circuit such as a ring modulating circuit.
2. DESCRIPTION OF THE BACKGROUND ART
FIG. 2 is a circuit diagram of a conventional differential amplifier circuit used as a ring modulating circuit which is a type of balanced modulating circuit. One end of a resistor 11 is connected to a negative input of an operational amplifier 1, and one end of a resistor 12 is connected to a positive input thereof. Resistors 13 and 14 connected in series are provided in a feedback path from the output of the operational amplifier I to the negative input thereof. Resistors 15 and 16 connected in series are provided in a reference voltage positive input path from the positive input of the operational amplifier 1 to a reference voltage VR. The resistors 11 to 16 have the same resistance.
An input signal SI to be modulated is applied to a signal input terminal 31, and a control signal SC is applied to a control signal input terminal 32. The output of the operational amplifier 1 is outputted from an output terminal 33 in the form of an output signal SO provided by the modulation of the input signal SI. An inverter 2 receives the control signal SC and inverts the logical level of the control signal SC to output an inverted control signal SC.
Analog switches 21 to 24, shown in FIG. 2, are of the same internal construction, which is shown in FIG. 3. Referring to FIG. 3, each analog switch includes PMOS and NMOS transistors 41 and 42 which share first and second electrodes serving respectively as an input A and an output B. A signal given from a control input C is applied to the gate of the PMOS transistor 41, and a signal given from a control input D is applied to the gate of the NMOS transistor 42.
The input A of the analog switch 21 is connected to the signal input terminal 31, and the output B thereof is connected to the other end of the resistor 11. The reference voltage VR is applied to the input A of the analog switch 22, and the output B of the analog switch 22 is connected to the other end of the resistor 11. The input A of the analog switch 23 is connected to the signal input terminal 31, and the output B thereof is connected to the other end of the resistor 12. The reference voltage VR is applied to the input A of the analog switch 24, and the output B of the analog switch 24 is connected to the other end of the resistor 12.
The control signal SC is applied to the control inputs C of the analog switches 21 and 24 and to the control inputs D of the analog switches 22 and 23. The inverted control signal SC is applied to the control inputs C of the analog switches 22 and 23 and to the control inputs D of the analog switches 21 and 24.
The ring modulating circuit is a differential amplifier circuit having such construction that varies the logical level of the control signal SC at the frequency of a predetermined carrier wave in order, signal SO, the modulated signal provided by the balanced modulation of the input signal SI
The amplification operation of the amplifier circuit will be described below. It is assumed that the reference voltage VR is 0 V. Reference character VI designates the voltage of the input signal SI; VO designates the voltage of the output signal SO; R designates the resistances of the resistors 11 to 16; and r designates the ON-resistances of the analog switches 21 to 24.
When the control signal SC is at the L level (or ground level), the analog switches 21 and 24 are on and the analog switches 22 and 23 are off. The input signal SI is applied to the negative input of the operational amplifier 1 through the resistor 11, and the reference voltage VR is applied to the positive input of the operational amplifier I through the resistor 12. As a result, the differential amplifier circuit functions as an inverting amplifier circuit, so that the output voltage VO is determined as: EQU VO=-{2R/(R+r)}.multidot.VI (1)
When the control signal SC is at the H level (or power supply voltage level), on the other hand, the analog switches 21 and 24 are off and the analog switches 22 and 23 are on. The input signal SI is applied to the positive input of the operational amplifier I through the resistor 2, and the reference voltage VR is applied to the negative input of the operational amplifier 1 through the resistor 11. As a result, the differential amplifier circuit functions as a noninverting amplifier circuit, so that the output voltage VO is determined as: EQU VO={2R/(R+r)}.multidot.VI (2)
From Formulas (1) and (2), the gain G of the differential amplifier circuit is: EQU G=2R/(R+r) (3)
The gain G is determined by the resistances R of the resistors 11 to 16 and the resistances r of the analog switches 21 to 24.
The conventional differential amplifier circuit used as the ring modulating circuit is constructed as above described. One of the parameters to determine the gain of the differential amplifier circuit is the ON-resistances r of the analog switches 21 to 24.
The ON-resistances of the PMOS and NMOS transistors 41 and 42 constituting the analog switches 21 to 24 depend mainly on a gate voltage and are inversely proportional to the square of the power supply voltage, varying with fluctuations in power supply voltage.
Therefore, the conventional differential amplifier circuit has a problem in that the gain thereof varies with fluctuations in power supply voltage.